1. Field of the Invention
The present invention relates generally to a driving circuit for IGBT (insulated-gate type bipolar transistor) and, more particularly, to a constant current circuit which controls a primary current of an ignition coil of an internal combustion engine ignition device used in various automotive vehicles.
2. Description of Related Art
As a primary current control circuits for ignition coils in internal combustion engine ignition devices, bipolar transistors have been used. That is, as illustrated in FIG. 8, NPN transistors 25 and 26 are connected in Darlington configuration. The collector current Ic of the Darlington transistor is detected by a primary current detecting resistor 27 to control the collector current Ic of the Darlington transistor. A comparator 28 compares the detection voltage across the primary current detecting resistor 27 with a comparison voltage and controls a base current control transistor (NPN transistor) 29 thereby to control the base current IB of the Darlington transistor. Thus, the electric current determined by the current amplification factor hFE of the Darlington transistor is produced. In order to suppress oscillation of the collector current Ic, an oscillation suppressing resistor 30 is connected between the base terminal of the Darlington transistor and the detection terminal side of the comparator 28. By feeding back the base voltage VBE to the detection terminal side of the comparator 28 through the oscillation suppressing resistor 30, the base current IB is suppressed from oscillation even when the current value detected by the primary current detecting resistor 27 is less than a predetermined value.
In such a circuit arrangement, however, since the bipolar transistors are used, the greater base current IB is required therefor. The electric power consumed by a base current control transistor (driving transistor) 31 which turns on and off in response to an ignition signal IGt and by a base current limiting resistor (driving resistor) 32 becomes greater and electric circuit components of greater power capacity are required. That is, the driving transistor 31 and the driving resistor 32 must amplify the current by the determined current amplifying factor hFE thereby to increase the collector current Ic of the bipolar transistors while the driving resistor 32 must have a resistance value of about several hundreds of ohms thereby to increase the collector current Ic of the bipolar transistors flowing through the driving resistor 32. Thus, both the driving resistor 32 and the driving transistor 31 require greater electric power capacity.
It may be conceived to replace the Darlington transistor in the constant current control circuit in FIG. 8 by an IGBT (insulated-gate type bipolar transistor) so that the electric power consumption at the driving stage may be reduced.
With the IGBT replaced for the Darlington transistor in FIG. 8, however, because the gate voltage of the IGBT is pulled down instantaneously by the NPN transistor 29, it is not possible to stabilize the gate voltage of the IGBT for keeping the collector current value at a constant value.